I don't think the freeview "tokens" expire. But as far as I can recall, if you use a freeview to view a private gallery then access to it expires when it would normally if you were to pay with credits. E.g. if it is 3 credits for 24 hours, as soon you use a freeview the clock is ticking for 24 hours from then. So if you want longer access then download the pics using firefox or something similar.
Hopefully that helps.